🔍 Key Concepts

• Definition of a sidereal day compared to a solar day • Role of the first point of Aries in celestial navigation and timekeeping • Meaning of the reference meridian (usually Greenwich) in defining time scales

💭 Think About

• Ask yourself: what must line up with what in the sky and on Earth to mark the start of a sidereal day? • Which object or point is used as the reference in sidereal time: the Sun or a fixed star/first point of Aries? • Among the choices, which one clearly ties the first point of Aries to a specific meridian on Earth rather than a calendar date or longitude value?

✅ Before You Answer

• Verify which event actually defines 0h sidereal time in celestial navigation texts • Check whether sidereal time is based on the Sun or on a fixed reference point among the stars • Confirm that the correct option links the first point of Aries to the observer’s/reference meridian, not to a calendar date or arbitrary longitude like 180°